Remove redundant references to IME Switcher vis

The taskbar and bubble bar were using both the visibility of the IME and
that of the IME Switcher button to determine whether these bars should
be stashed. However, the IME Switcher button will never be visible while
the IME is not visible, so using its visibility is redundant.

This changes the stashing logic to only take into account the IME
visibility. Note, the IME is also considered visible even when a
hardware keyboard is connected, regardless of any UI being visible or
not. This visibility state determines various system behaviours.

Flag: EXEMPT refactor
Bug: 366129400
Test: open the IME and IME Switcher menu on large screen devices;
  observe taskbar behaviour
Change-Id: Ibcd16896582ca575538d8c1c3d2ab879090d075c